1. Field of the Invention
The present invention relates to an electronic endoscope apparatus which performs photoelectric conversion of an optical image formed by an image pickup optical system with an image pickup device to output an electronic image signal.
2. Description of the Related Art
Recently, in a medical field or an industrial field, an electronic endoscope apparatus has been widely and generally put to practical use which is provided with an image pickup unit constituted by an image pickup optical system, an image pickup device and the like, at a distal end portion of an elongated insertion portion, and configured to be capable of displaying an image of a subject on a display device and observing the image by performing photoelectric conversion of an optical image formed by the image pickup optical system with the image pickup devices and outputting an electronic image signal to the display device. An endoscope apparatus for the medical field is configured such that observation and the like inside a body cavity can be performed by inserting the insertion portion into a body from an oral cavity, an anus or the like. Further, an endoscope apparatus for the industrial field is configured such that internal observation and the like can be performed by inserting the insertion portion into a pipework of plant facilities, such as a boiler, an inside of an engine or the like.
There is a remarkable tendency that the number of pixels of an image pickup device adopted for an endoscope apparatus of this kind is increased for the purpose of improving image quality and resolution of an acquired image. Therefore, it becomes possible to improve resolution and obtain a high-quality image by increase in the number of pixels of an image pickup device, but there is a tendency that depth of field (depth of observation) is shallower.
For example, FIGS. 15(A) and 15(B) show conceptual diagrams of an image pickup unit of a conventional endoscope apparatus. Between the diagrams, FIG. 15(A) is a conceptual diagram showing a state in which a subject at a near point is focused (a focused state). Further, FIG. 15(B) is a conceptual diagram showing a state in which a subject at a far point is focused (a focused state).
As shown in FIGS. 15(A) and 15(B), an image pickup unit 109 is mainly configured with an image pickup objective lens 133 which forms optical images of subjects 101(e and k) and an image pickup device 131 which receives an optical image formed by the image pickup objective lens 133 and performs photoelectric conversion to generate an image signal. An optical axis O of the image pickup objective lens 133 is set so as to almost correspond to a center of a light receiving plane 131b of the image pickup device 131. A color filter which optically performs color separation is arranged on a front of a light receiving portion 131a of the image pickup device 131.
It is assumed that, in the image pickup unit 109 in such a configuration, as shown in, for example, FIG. 15(A) a setting is made so that an optical image of the subject 101k existing at a near point (a short-distance position) is formed on the light receiving plane 131b of the image pickup device 131 in a focused state. A distance, that is, an optical path length from the image pickup objective lens 133 to the light receiving plane 131b (a plane denoted by reference symbol k in FIG. 15(A)) of the image pickup device 131 at this time, is denoted by reference symbol OPL1. In this setting state, an optical image of the subject 101e existing at a far point (a long-distance position) exists at a position denoted by reference symbol k.
On the other hand, in order to cause the optical image of the subject 101e existing at the far point (the long-distance position) to be in a focused state on the light receiving plane 131b of the image pickup device 131, it is necessary to move the light receiving plane 131b of the image pickup device 131 from the state shown in FIG. 15(A) to a position near to the image pickup objective lens 133 in a direction along the optical axis O, that is, a position corresponding to a plane denoted by reference symbol e as shown in FIG. 15(B). A distance from the image pickup objective lens 133 to the light receiving plane 131b (a plane denoted by reference symbol k in FIG. 15(B)) of the image pickup device 131 is an optical path length OPL2. That is, the near-point optical path length OPL1>the far-point optical path length OPL2 is satisfied.
Therefore, for the conventional endoscope apparatus, various devices for configuring the endoscope apparatus such that an image can be acquired in which a focused state is obtained at a near point and a far point without providing the focus adjustment mechanism are proposed, for example, by Japanese Patent Application Laid-Open Publication No. 11-197097.
An endoscope apparatus disclosed by Japanese Patent Application Laid-Open Publication No. 11-197097 and the like is such that an optical path of a light flux which has passed through an objective lens is separated by a prism so that the separated light fluxes are caused to enter an image pickup device for a far point and an image pickup device for a near point set to different optical path lengths, and an image signal outputted from each of the image pickup device is acquired. Due to the configuration, in the endoscope apparatus disclosed by Japanese Patent Application Laid-Open Publication No. 11-197097, a plurality of images focused at different distances can be simultaneously picked up and displayed.